Having used NFT for some years now, I've found that the best watering regime is 15mins on 15 mins off. Another tip is: clones, seedlings must be VERY well rooted when first introduced to the system, or you'll find they can't cope to well. Start them off gently...ie 30 mins on 30 off, to coax the roots out first. After that, NFT is reasonably trouble free.